294 OSTREAD^. 



the lower valve. The painting of the upper valve is varia- 

 ble, yet usually consists of bright red markings upon a 

 pale or yellowish-white ground, exhibiting some diversity 

 of arrangement, and very often displaying a mottled ap- 

 pearance, with frequently, too, a few narrow interrupted 

 rays of blood-red and occasionally opaque white amorphous 

 patches. Sometimes the pattern is composed of white 

 and scarlet zigzags disposed somewhat concentrically upon 

 a pale horn-coloured ground. The lower valve is either 

 whitish or faintly exhibits the hues of the superior one. 

 Neither stria?, ribs, radiating folds, nor microscopic chasing 

 are visible, the entire surface being perfectly smooth, with, 

 at most, faint indications of concentric lineoles. The sides 

 are unequal ; the anterior is the more produced, and 

 slightly the narrower at its projecting termination ; the pos- 

 terior is the shorter and the more broadly rounded at its 

 extremity. The auricles are rather large and not very 

 unequal, have their lateral edges convex, and their upper 

 angles, which are both of them rounded off, rather more 

 than riffht angles. The hinder ears are not well defined 

 at their commencement : that of the upper valve, although 

 larger in area, scarcely surpasses the other in length ; the 

 front one of the superior valve rather curls upward at its 

 top. The auricular sinus is small and rather shallow, but 

 acutely angular below, and well indicated. The hinge- 

 margin is very long, being considerably more than half 

 the length of the shell. The extension of the posterior 

 auricle causes the lateral outline of that side of the shell 

 to appear peculiarly abrupt and but little indented, the 

 outline of the ear being united to that of the body in 

 almost a continuous curve. 



Few of our British specimens measure much more than 

 a quarter of an inch in diameter. The P. Greenlandicus 



